With regard to mobile communication systems, such as based on a GSM (Global System for Mobile communications), UMTS (Universal Mobile Telecommunication Systems), WCDMA (Wideband Code Division Multiple Access) and/or CDMA 2000 standard there is a permanently increasing need to design re-configurable multiband and/or multistandard mobile communication systems. This for example, is based on the fact, that on the one hand side more and more frequency bands are opened up and being licensed for mobile communication and on the other hand side, several standards are provided in parallel, between which the customers and/or the network operator would like to select.
However, until today, for every new frequency band an individual radio has to be designed that forces a complex infrastructure and terminal vendor to a great variety of products, in particular in terms of base stations and/or user terminals, such as handsets adapted to be used within these communication systems.
As a consequence, there is a need both for new receiving and/or transmitting architectures in particular for ensuring re-configurability in terms of frequency band and system standards and for improved flexible circuits in particular for optimization, tuning and/or matching purposes.
Moreover, even with regard to other high frequency receiving/transmitting-systems for data transmission there is an increased need for receiver and/or transmitter chains, which provide flexibility in use by re-configurability and by tuning and/or matching functionality for optimization.
Based on the above needs, in particular with regard to upcoming multiband and/or multistandard systems, one of the major task is to tune the respective incorporated voltage controlled oscillators (VCO) within the desired frequency band and to match those VCO's to the resonance circuit (or tank-circuit) for optimization. Accordingly, even VCO's must be re-configurable and must have tuning and matching facilities for optimization.
The most common solution to implement VCO's for several frequency bands is however to design a lot of separate VCO-circuits for every band and to switch them on and off via a supply or control voltage.
However, designing a lot of separate VCO-circuits for every band is a very expensive solution, since, for example, every VCO typically comprises a tank-circuit, a tuning device, such as a varactor which has to be added separately to the tank circuit for obtaining tuning functionality, and a negative impedance for tuning on the oscillation. Moreover, the respective design is very difficult due to the fact that a trade-off between some conflicting requirements has to be made. Such conflicting requirements may include the reduction of wideband phase noise, of low frequency phase noise near the carrier and/or of so called “popcorn-noise” or Flicker noise, the stabilization of the oscillation under all conditions, the provision of linearity of frequency versus the control voltage and the provision of a large tuning range.
Furthermore, for determining the negative resistance conventionally two additional fixed capacitors have to be arranged between the basis, emitter and ground of the incorporated transistor, thereby causing an increased design trade-off. As a consequence, enabling a good tuning range usually results in a poor noise performance, whereas a bad VCO tuning range may cause a good noise behavior.